This invention broadly relates to a method for forming at least one ceramic layer by manually pressing at least one ceramic tape, while heated, against the surface of a turbine component.
Components located in certain sections of gas turbine engines, such as the turbine, combustor, augmentor, exhaust nozzle, etc., are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These ceramic layers, often referred to as thermal barrier coatings (TBCs), have low thermal conductivity, need to strongly adhere to the component, and need to remain adherent throughout many heating and cooling cycles.
Coating systems capable of satisfying the above requirements typically include a metallic bond coat layer that adheres the thermal-insulating ceramic layer to the metal substrate of the component. Metal oxides, such as zirconia, partially or fully stabilized with yttria, magnesia or other stabilizer metal oxides, have been used as the materials in this thermal-insulating ceramic layer. This ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EB-PVD), to provide a strain-tolerant columnar grain structure. Bond coat layers are typically formed of an oxidation-resistant diffusion coating material such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant overlay metal alloy material such as MCrAlY (where M is typically iron, cobalt, nickel, or a combination thereof). Aluminide coatings are distinguished from MCrAlY coatings, in that the former are intermetallic (diffusion) coatings, while the latter are metallic solid solution coatings.
While coating systems of the type described above are widely used, the requirement that the coating system remain adherent to the metal substrate surface through many heating and cooling cycles is particularly demanding because the coefficient of thermal expansion (CTE) of ceramic materials is usually significantly lower than that of the superalloy metals typically used as the substrate of turbine engine components. Such differences in CTE, in combination with oxidation of the underlying bond coat layer or metal substrate, can eventually lead to spallation and partial or complete loss of the coating system. See background section of commonly-assigned U.S. Pat. No. 6,485,590 (Ivkovich, Jr. et al), issued Nov. 26, 2002.
An additional desired characteristic for a coating system of a gas turbine engine component is for the outermost surface of the coating system to be extremely smooth in order to promote the aerodynamics of the component surface. While relatively smooth ceramic coatings can be produced by spraying or PVD techniques, smoother surface finishes would be desirable. In general, the techniques described above tend to produce ceramic coatings that are relatively porous, which is advantageous in achieving a lower coefficient of thermal conduction. See background section of commonly-assigned U.S. Pat. No. 6,485,590 (Ivkovich, Jr. et al), issued Nov. 26, 2002.
However, porosity in ceramic coatings can also promote surface roughness (Ra). For example, ceramic coatings deposited by PVD techniques generally have a surface roughness of about 60 microinches (about 1.5 micron) Ra and higher. Those deposited by APS and LPPS techniques typically have an even higher surface roughness of from about 260 to about 400 microinches (from about 6.6 to about 10.2 microns) Ra. Ceramic coatings deposited by conventional spray methods on components with complex geometries (e.g., curved surfaces) are further prone to such surface flaws as shadowing effects (i.e., thin or beaded regions caused by partial masking due to component shape) and slumping (i.e., thicker regions formed by movement of material to low portions of a component due to gravity). See background section of commonly-assigned U.S. Pat. No. 6,485,590 (Ivkovich, Jr. et al), issued Nov. 26, 2002.
Accordingly, it would be desirable to be able to form one or more ceramic layers on the surface of gas turbine engine components that: (1) are adherent to the component surface through many heating and cooling cycles; and (2) can be denser and smoother for improved aerodynamic performance.